sys-libs/glibc: disable c++ detection for cross-case, bug #683074

sys-libs/glibc: disable c++ detection for cross-case, bug #683074

https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=561b0bec4448f0302cb4915bf67c919bde4a1c57
in unstream glibc repo added optional dependency on CXX compiler.

Unfortunately ./configure tries hard to pass some g++ as CXX:
either ${CTARGET}-g++ (ok) or ${CBUILD}+g++ (bad).

During toolchain bootstrap ${CTARGET}-g++ does not yet exist
as we only have stage1-gcc without c++ (or libc) support.

To restore toolchain bootstrap we force-disable c++ detection
by setting cache variable:
    libc_cv_cxx_link_ok=no

c++ is force disables only for cross-compilation case
(CHOST != CTARGET). Cross-building case (CHOST = CTRAGET)
is unchanged as we should already have c++ capable toolchain
by then.

Reported-by: hanetzer@startmail.com
Reported-by: rhn
Closes: https://bugs.gentoo.org/683074
Package-Manager: Portage-2.3.63, Repoman-2.3.12
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>